Tolkien said he wanted them to use Stealth instead of brute force thats why they went with a burglar instead of a warrior. So a small band of dwarfs would be better than a big army of them. surely the dragon would see a large force coming and obliterate them before they even got close where as a small group was able to get into Erobor. Granted had they not got into Erobor and ticked off Smaug he wouldn't have flown out and got himself killed by Bard. it all worked out for the best as it was written